The present invention relates to a new and improved construction of a smoke detector which is of the type containing a radiation source which transmits radiation in a predetermined spatial region, and further contains at least one radiation receiver arranged externally of a direct radiation region of the radiation source. There is inputted to the radiation receiver the radiation which is scattered at particles emanating from a combustion process and located in the radiation region.
A smoke detector of this general type is known to the art, for instance from Swiss Pat. No. 592,932. This smoke detector contains an evaluation circuit which triggers an alarm upon attaining a predetermined signal peak of the received scattered radiation. There prevails a fixedly set or adjusted smoke concentration at which there is triggered the alarm.
In order to be able to accommodate the smoke detector to different ambient conditions it is necessary to undertake a sensitivity setting or adjustment. This can be achieved, for instance, by changing an alarm threshold. However, it is frequently advantageous to be able to separately adjust the individual fire alarms or smoke detectors. In the case of ionization smoke detectors, Swiss Pat. No. 468,683 proposes an electrical circuit arrangement for accomplishing sensitivity changes, wherein the adjustment of the response threshold of a field-effect transistor serving as an amplifier and threshold detector is accomplished by means of a potentiometer constituting part of a voltage divider, by means of which there can be biased the source-electrode. Even if the adjustment or setting is not continuously undertaken by means of a potentiometer, rather by means of a switch with which there can be connected into the current circuit fixed resistances of different magnitudes, such electrical adjustment or setting possibilities are prone to disturbance and not resistant to corrosion.